Hemodynamically unstable ventricular arrhythmia detection

ABSTRACT

An implantable medical device system includes a pacemaker and an implantable cardioverter defibrillator (ICD). The pacemaker is configured to confirm a hemodynamically unstable rhythm based on an activity metric determined from an activity sensor signal after detecting a ventricular tachyarrhythmia and withhold anti-tachycardia pacing (ATP) pulses in response to confirming the hemodynamically unstable rhythm. The pacemaker may deliver ATP when a hemodynamically unstable rhythm is not confirmed based on the activity metric. The ICD is configured to detect the ATP and withhold a shock therapy in response to detecting the ATP in some examples.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. Ser. No. 16/056,676, (U.S.Publication No. 2018/0339163), filed Aug. 7, 2018, entitled“HEMODYNAMICALLY UNSTABLE VENTRICULAR ARRHYTHMIA DETECTION,” which is aDivisional of U.S. Ser. No. 14/657,001, now U.S. Pat. No. 10,052,494,entitled “HEMODYNAMICALLY UNSTABLE VENTRICULAR ARRHYTHMIA DETECTION”,filed Mar. 13, 2015, which claims the benefit of U.S. ProvisionalApplication No. 62/095,971, entitled “HEMODYNAMICALLY UNSTABLEVENTRICULAR ARRHYTHMIA DETECTION”, filed on Dec. 23, 2014. The contentof all of which are incorporated by reference in their entirety.

TECHNICAL FIELD

The disclosure relates to implantable medical device systems includingan implantable pacemaker and implantable cardioverter defibrillator andmethods for detecting ventricular tachyarrhythmia and controllingtachyarrhythmia therapies delivered by the pacemaker and ICD.

BACKGROUND

A variety of implantable medical devices (IMDs) for delivering atherapy, monitoring a physiological condition of a patient or acombination thereof have been clinically implanted or proposed forclinical implantation in patients. Some IMDs may employ one or moreelongated electrical leads carrying stimulation electrodes, senseelectrodes, and/or other sensors. Other IMDs may incorporate electrodesand/or other sensors along or within a housing of the IMD that enclosescircuitry and electronic components of the IMD.

IMDs may deliver therapy to and/or monitor conditions of a variety oforgans, nerves, muscle or tissue, such as the heart, brain, stomach,spinal cord, pelvic floor, or the like. Some IMDs, such as cardiacpacemakers monitor a patient's heart activity and provide therapeuticelectrical stimulation to the heart of the patient via electrodescoupled to the pacemaker. The electrical stimulation provided by the IMDmay include signals such as pacing pulses to address abnormal cardiacrhythms such as bradycardia, tachycardia and fibrillation.

An IMD may sense signals representative of intrinsic depolarizations ofthe heart and analyze the sensed signals to identify normal or abnormalrhythms. Upon detection of an abnormal rhythm, the device may deliver anappropriate electrical stimulation signal or signals to restore ormaintain a more normal rhythm. For example, an IMD may deliverbradycardia pacing, anti-tachycardia pacing (ATP), or cardioversion ordefibrillation shocks to the heart upon detecting an abnormal rhythm.

In some cases, the IMD senses a signal representative of the metabolicdemand of the patient in order to provide cardiac pacing at a rateintended to meet the metabolic demand of the patient. For example, anindication of the patient's physical activity level may be determinedfrom an accelerometer signal in order provide rate responsive pacing todynamically maintain a heart rate that meets the metabolic demand of thepatient.

SUMMARY

In general, the disclosure is directed to techniques for controllingelectrical stimulation therapies delivered by an implantable medicaldevice system that includes a cardiac pacemaker capable of deliveringanti-tachycardia pacing. A pacemaker operating in accordance with thetechniques disclosed herein confirms a hemodynamically unstable rhythmbased on an activity sensor signal that comprises heart motion signals.The pacemaker may withhold ATP in response to confirming the unstablerhythm based on the activity sensor signal and deliver ATP in responseto not confirming a hemodynamically unstable rhythm based on theactivity sensor signal. The system may include an ICD configured todetect ATP delivered by the pacemaker, withhold a shock therapy when ATPdelivered by the pacemaker is detected, and deliver a shock therapy whenventricular tachyarrhythmia is detected and ATP is not detected by theICD.

In one example, the disclosure provides a method performed by an IMDsystem comprising: sensing a first cardiac electrical signal by anelectrical sensing module of a cardiac pacemaker; detecting, by thepacemaker, a ventricular tachyarrhythmia from the first cardiacelectrical signal; determining an activity metric from an activitysensor signal received by a control module of the pacemaker; confirminga hemodynamically unstable rhythm based on the activity metric; andwithholding anti-tachycardia pacing (ATP) by the pacemaker in responseto confirming the hemodynamically unstable rhythm.

In another example, the disclosure provides an implantable medicaldevice (IMD) system comprising an activity sensor and a cardiacpacemaker. The activity sensor is configured to produce a signalcorrelated to patient activity and comprising heart motion signals. Thepacemaker comprises a first sensing module configured to receive acardiac electrical signal; a pulse generator configured to generate anddeliver pacing pulses to a patient's heart via a pair of electrodes; anda control module coupled to the sensing module, the pulse generator andthe activity sensor and configured to detect a ventriculartachyarrhythmia from the first cardiac electrical signal, determine anactivity metric from an activity sensor signal; confirm ahemodynamically unstable rhythm based on the activity metric, andcontrol the pulse generator to withhold anti-tachycardia pacing (ATP) inresponse to confirming the hemodynamically unstable rhythm.

In another example, the disclosure provides a non-transitory, computerreadable storage medium storing a set of instructions that, whenexecuted by control circuitry of an implantable medical device system,cause the system to sense a first cardiac electrical signal by anelectrical sensing module of a cardiac pacemaker; detect, by thepacemaker, a ventricular tachyarrhythmia from the cardiac electricalsignal; determine an activity metric from an activity sensor signalreceived by a control module of the pacemaker; confirm a hemodynamicallyunstable rhythm based on the activity metric; and withholdanti-tachycardia pacing (ATP) by the pacemaker in response to confirmingthe hemodynamically unstable rhythm.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an IMD system used to sensecardiac electrical signals in a patient and provide therapies to thepatient's heart.

FIG. 2 is a conceptual diagram of intracardiac pacemaker shown in FIG.1.

FIG. 3 is a functional block diagram of an example configuration of thepacemaker shown in FIG. 2.

FIG. 4 is a schematic diagram of the ICD shown in FIG. 1 according toone example.

FIG. 5 is a plot of a sensor-indicated rate (SIR) transfer function thatmay be used by the pacemaker of FIG. 1 for controlling therapiesdelivered to a patient's heart according to one example.

FIG. 6 is a flow chart of a process performed by the IMD system shown inFIG. 1 according to one example.

FIG. 7 is a timing diagram illustrating the operations of IMD system ofFIG. 1 according to one example.

FIG. 8 is a timing diagram illustrating the operations of the IMD systemof FIG. 1 according to another example.

FIG. 9 is a flow chart of a method for controlling shock delivery by theICD of FIG. 1 in the presence of the intracardiac pacemaker.

DETAILED DESCRIPTION

In general, an implantable medical device (IMD) system is disclosedherein that includes a pacemaker configured to deliver cardiac pacingtherapies, detect tachyarrhythmia and confirm a hemodynamically unstabletachyarrhythmia based at least in part on an activity sensor signal,e.g., an accelerometer. The pacemaker may be configured to be implantedwholly in a chamber of a patient's heart in some examples. The pacemakeractivity sensor is subjected to heart motion so that the activity sensorsignal comprises heart motion signals. Therapies delivered by thepacemaker are controlled based at least in part on the activity sensorsignal. For example, techniques disclosed herein enable the intracardiacpacemaker to confirm the detection of a hemodynamically unstable rhythmbased on the activity sensor signal meeting unstable rhythm detectioncriteria. The pacemaker withholds ATP in response to confirming thehemodynamically unstable rhythm. The IMD system may further include anICD configured to detect and treat abnormal rhythms via electrodescoupled to the ICD. The ICD may be configured to detect ATP delivered bythe pacemaker and withhold a cardioversion/defibrillation (CV/DF) shockin response to detecting the ATP.

Shockable rhythms refer to abnormal heart rhythms for which a shocktherapy is delivered to one or both of the ventricles. Shockable rhythmsmay include ventricular tachycardia and ventricular fibrillation (VF)and are generally referred to herein as “ventricular tachyarrhythmia” or“VT.” Shockable rhythms generally pose an immediate danger to thepatient because they are hemodynamically unstable and therapy is neededin order to promote the well-being and safety of the patient. A shocktherapy delivered in response to detecting a shockable rhythm generallyincludes at least one a high-voltage shock pulse, which may be in therange of at least 10 Joules and up to 35 Joules for transvenous leadsystems carrying intracardiac cardioversion/defibrillation electrodesand in the range of at least 65 Joules and up to 80 Joules forsubcutaneous lead systems carrying extracardiaccardioversion/defibrillation electrodes.

In some instances, VT may be initially treated using an alternativeelectrical stimulation therapy before attempting a shock therapy,particularly if the VT is hemodynamically stable. The electricalstimulation therapy may be a low-voltage pacing therapy such asanti-tachycardia pacing (ATP), which is relatively painless to thepatient compared to a shock therapy. ATP may successfully terminate aVT. If ATP fails, the rhythm may be treated with a shock to terminatethe arrhythmia.

Non-shockable rhythms, on the other hand, refer to abnormal or normalheart rhythms that do not require a shock therapy to be delivered toeither of the ventricles. Non-shockable cardiac rhythms may includesupra-ventricular tachycardia (SVT), which includes sinus tachycardia,atrial tachycardia (AT), atrial fibrillation (AF), atrial flutter,atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricularreciprocating tachycardia (AVRT), or the like. Non-shockable rhythms donot generally pose an immediate danger to the patient and may gountreated, i.e., no shock therapy is delivered to the heart.

FIG. 1 is a conceptual diagram illustrating an IMD system 10 used tosense cardiac electrical signals in patient 12 and provide therapy toheart 26. IMD system 10 includes an intracardiac pacemaker 100 and ICD14 coupled to an extravascular defibrillation lead 16. ICD 14 isimplanted subcutaneously on the left side of patient 12. Defibrillationlead 16, which is connected to ICD 14, extends medially from ICD 14toward sternum 22 and xiphoid process 20 of patient 12. At a locationnear xiphoid process 20 defibrillation lead 16 bends or turns andextends subcutaneously superior, substantially parallel to sternum 22.Defibrillation lead 16 may be implanted such that lead 16 is offsetlaterally to the left side of the body of sternum 22 (i.e., towards theleft side of patient 12), offset to the right of sternum 22 or oversternum 22.

Defibrillation lead 16 includes a defibrillation electrode 24, which maybe an elongated coil electrode, and a pair of sensing electrodes 28 and30. Defibrillation lead 16 is placed along sternum 22 such that atherapy vector between defibrillation electrode 24 and the housing 15 ofICD 14 is substantially across one or both ventricles of heart 26. Inother examples, another electrode along lead 16 or along a second leadmay be used in combination with defibrillation electrode 24 fordelivering a shock therapy.

In the example illustrated in FIG. 1, lead 16 is implantedsubcutaneously, e.g., between the skin and the ribs or sternum. Lead 16is advanced suprasternally remaining external to the thoracic cavity. Inother embodiments, lead 16 may be advanced substernally or withinribcage 32, i.e., intra-thoracically. For example, lead 16 may beimplanted at least partially in a substernal location. In such aconfiguration, a portion of lead 16 may extend subcutaneously from ICD14 toward sternum 22 and at least a portion of lead 16 is advanced underor below the sternum in the mediastinum and, more particularly, in theanterior mediastinum. The anterior mediastinum is bounded laterally bypleurae, posteriorly by pericardium, and anteriorly by sternum 22. Lead16 may be at least partially implanted in other intrathoracic locations,e.g., locations in the region around but not necessarily in directcontact with the outer surface of heart 26. These other intrathoraciclocations may include in the mediastinum but offset from sternum 22, inthe superior mediastinum, in the middle mediastinum, in the posteriormediastinum, in the sub-xiphoid or inferior xiphoid area, near the apexof the heart, or other location in direct contact or not in directcontact with heart 26 and not subcutaneous.

Although ICD 14 is illustrated as being implanted near a midaxillaryline of patient 12, ICD 14 may also be implanted at other subcutaneouslocations on patient 12, such as further posterior on the torso towardthe posterior axillary line, further anterior on the torso toward theanterior axillary line, in a pectoral region, or at other locations ofpatient 12. In instances in which ICD 14 is implanted pectorally, lead16 may follow a different path, e.g., across the upper chest area andinferior along sternum 22. When the ICD 14 is implanted in the pectoralregion, lead 16 or a second lead including a defibrillation electrodemay extend along the left side of the patient such that a defibrillationelectrode is located along the left side of the patient to function asan anode or cathode of a therapy vector for defibrillating heart 26.

ICD 14 includes a housing 15 that forms a hermetic seal that protectselectronic circuitry and other components within ICD 14. The housing 15of ICD 14 may be formed of a conductive material, such as titanium orother biocompatible conductive material or a combination of conductiveand non-conductive materials. In some instances, the housing 15functions as an electrode (sometimes referred to as a housing electrodeor “can” electrode) that is used in combination with one of electrodes24, 28 and 30 to deliver a therapy to heart 26 or to sense electricalactivity of heart 26.

ICD 14 may also include a connector assembly 13 (also referred to as aconnector block or header) for receiving a proximal connector (notillustrated) of lead 16. Connector assembly 13 includes electricalfeedthroughs through which electrical connections are made betweenconductors within defibrillation lead 16 and electronic componentsincluded within the housing.

Defibrillation lead 16 includes a lead body having a proximal end thatincludes a connector configured to connect to ICD 14 (via connectorassembly 13) and a distal end that includes electrodes 24, 28 and 30.The lead body of defibrillation lead 16 may be formed from anon-conductive material, including silicone, polyurethane,fluoropolymers, mixtures thereof, and other appropriate materials, andshaped to form one or more lumens within which the one or moreconductors (not illustrate) each extend to respective ones of electrodes24, 28 and 30.

When the connector at the proximal end of defibrillation lead 16 isconnected to connector assembly 13, the respective conductorselectrically couple to circuitry of ICD 14, such as a therapy module anda sensing module via connections in connector assembly 13, includingassociated feedthroughs. The electrical conductors transmit electricalstimulation pulses from a therapy module within ICD 14 to one or more ofelectrodes 24, 28 and 30 and transmit sensed electrical signals from oneor more of electrodes 24, 28 and 30 to the sensing module within ICD 14.Although defibrillation lead 16 is illustrated as including threeelectrodes 24, 28 and 30, defibrillation lead 16 may include more orfewer electrodes. For example, two or more sensing electrodes may beincluded for sensing an ECG signal.

ICD 14 may sense electrical activity of heart 26 via one or more sensingvectors that include combinations of electrodes 28 and 30 and housing15. For example, ICD 14 may obtain electrical signals, e.g., ECGsignals, using a sensing vector between electrodes 28 and 30, betweenelectrode 28 and housing 15, between electrode 30 and housing 15, or anycombination thereof. In some instances, ICD 14 may even sense cardiacelectrical signals using a sensing vector that includes defibrillationelectrode 24, such as a sensing vector between defibrillation electrode24 and one of electrodes 28 and 30, or a sensing vector betweendefibrillation electrode 24 and the housing 15.

ICD 14 and pacemaker 100 are each configured to detect ventriculararrhythmias from cardiac electrical signals received via respectiveelectrodes coupled to respective sensing modules within each of the ICD14 and pacemaker 100. The cardiac electrical signals include cardiacevent signals attendant to the depolarization (e.g., R-waves) and therepolarization (e.g., T-waves) of the ventricles.

ICD 14 analyzes sensed ECG signals to detect ventriculartachyarrhythmias (VT), and in response to detecting VT may generate anddeliver an electrical therapy to heart 26. For example, ICD 14 maydeliver one or more defibrillation shocks via a therapy vector thatincludes defibrillation electrode 24 and the housing 15. In someinstances, IMD system 10 may be configured to deliver one or more pacingtherapies prior to or after delivery of a defibrillation shock by ICD14, such as ATP or post shock pacing.

Pacemaker 100 may be configured to confirm a hemodynamically unstablerhythm based on an activity sensor signal after detecting VT based on acardiac electrical signal. If the detected VT is confirmed as ahemodynamically unstable rhythm, the pacemaker 100 withholds ATP,allowing ICD 14 to deliver a necessary CV/DF shock. If the detected VTis not confirmed to be a hemodynamically unstable rhythm by pacemaker100 based on the activity sensor signal, pacemaker 100 delivers ATP. ICD14 is configured to detect the ATP delivered by pacemaker 100 from anECG signal received from electrodes 24, 28, 30 and/or housing 15 andwithhold a shock until ATP is no longer detected and VT is still beingdetected.

Alternatively, pacemaker 100 may transmit a wireless telemetry signal toICD 14 to signal ICD 14 to withhold a shock therapy until an ATP therapydelivered by pacemaker 100 is completed. In other examples, ICD 14 maytransmit a communication signal to pacemaker 100 to cause pacemaker 100to deliver a pacing therapy, such as ATP or post-shock bradycardiapacing, e.g., as generally disclosed in U.S. Pat. No. 8,744,572(Greenhut, et al.), incorporated herein by reference in its entirety.

In the example shown, pacemaker 100 is a transcatheter, intracardiacpacemaker adapted for implantation wholly within a heart chamber, e.g.,wholly within the RV. Pacemaker 100 may be positioned along anendocardial wall of the RV, e.g., near the RV apex, however otherlocations within or along heart 26 are possible including epicardiallocations. Pacemaker 100 is configured to sense an intracardiacelectrogram (EGM) signal via one or more electrodes on the outer housingof the pacemaker 100 and produce electrical stimulation pulses, i.e.,pacing pulses, delivered to heart 26 via the one or more electrodes onthe outer housing of the pacemaker 100.

In other embodiments, pacemaker 100 may be positioned in the leftventricle (LV) of heart 26. In some cases, IMD system 10 mayadditionally include an intracardiac pacemaker implantable in an atrialchamber for sensing atrial electrical signals and delivering atrialpacing pulses. Techniques disclosed herein are not limited to apacemaker implantable within a heart chamber. In other examples,pacemaker 100 could be implantable inside or outside heart 26. In eachof these examples, pacemaker 100 includes a motion-based patientactivity sensor that is subjected to the motion of the beating heart 26such that an activity sensor signal includes heart motion signals.

Pacemaker 100 and ICD 14 are each capable of bidirectional wirelesscommunication with an external device 40. External device 40 is oftenreferred to as a “programmer” because it is typically used by aphysician, technician, nurse, clinician or other qualified user forprogramming operating parameters in pacemaker 100 and ICD 14. Operatingparameters, such as sensing and therapy delivery control parameters, maybe programmed into pacemaker 100 and ICD 14 using external device 40.External device 40 may be located in a clinic, hospital or other medicalfacility. External device 40 may alternatively be embodied as a homemonitor or a handheld device that may be used in a medical facility, inthe patient's home, or another location. Aspects of external device 40may generally correspond to the external programming/monitoring unitdisclosed in U.S. Pat. No. 5,507,782 (Kieval, et al.), herebyincorporated herein by reference in its entirety.

External device 40 includes a processor 52, memory 53, user display 54,user interface 56 and telemetry module 58. Processor 52 controlsexternal device operations and processes data and signals received frompacemaker 100 and ICD 14. External device 40 may be used to programoperating parameters, such as sensing control parameters,tachyarrhythmia detection control parameters, and therapy deliverycontrol parameters used by pacemaker 100 and ICD 14. Processor 52 mayprovide user display 54 with data for generating a graphical userinterface to a user for selecting and programming control parameters.

External device 40 may display other data and information relating topacemaker 100 and ICD 14 functions to a user for reviewing IMD systemoperation and programmed parameters as well as EGM and ECG signals orother physiological data that is retrieved from pacemaker 100 and/or ICD14 during an interrogation session.

In some examples, pacemaker 100 may store activity sensor signal data inconjunction with EGM signal data to be transmitted to external device40. External device 40 may provide the data to user display 54 forgenerating a display of activity sensor signal data and cardiacelectrical signal data during a detected arrhythmia. The user mayevaluate the reliability of the activity sensor signal data, e.g.,activity metrics determined from the sensor signal, for use inconfirming a hemodynamically unstable rhythm and making ATP therapydelivery decisions by pacemaker 100. The user display 54 may generate adisplay of activity metric distribution for review by a user.

As described below, the distribution of activity metrics determined overtime from the activity sensor signal may be used in establishing anunstable rhythm detection threshold and/or in verifying reliability ofthe activity sensor signal in confirming a hemodynamically unstablerhythm. In some cases, the user may select and program an unstablerhythm threshold based on a display of the activity metric distribution.The activity metric unstable rhythm threshold is used by the pacemaker100 in confirming a hemodynamically unstable rhythm and making adecision to deliver ATP or not in response to detecting VT.

User interface 56 may include a mouse, touch screen, keyboard and/orkeypad to enable a user to interact with external device 40 to initiatea telemetry session with pacemaker 100 or ICD 14 for retrieving datafrom and/or transmitting data to pacemaker 100 or ICD 14 for selectingand programming desired sensing and therapy delivery control parameters.

Telemetry module 58 is configured for bidirectional communication withseparate implantable telemetry modules included in pacemaker 100 and ICD14. Telemetry module 58 establishes a wireless radio frequency (RF)communication link 42 or 44 with a targeted one of ICD 14 or pacemaker100, respectively, using a communication protocol that appropriatelyaddresses the targeted ICD 14 or pacemaker 100. An example RF telemetrycommunication system that may be implemented in system 10 is generallydisclosed in U.S. Pat. No. 5,683,432 (Goedeke, et al.), herebyincorporated herein by reference in its entirety. Telemetry module 58 isconfigured to operate in conjunction with processor 52 for sending andreceiving data via communication link 42 or 44. Communication links 42and 44 may be established between the respective ICD 14 or pacemaker 100and external device 40 via a radio frequency (RF) link in the MedicalImplant Communication Service (MICS) band, Medical Data Service (MEDS)band, BLUETOOTH® or Wi-Fi.

Telemetry module 58 may be capable of bi-directional communication withICD 14 or pacemaker 100 over a wide range of distances, e.g., up toapproximately 10 meters. In other examples, telemetry communicationpacemaker 100 and ICD 14 may require the use of a programming headplaced in proximity of the respective pacemaker 100 or ICD 14 tofacilitate data transfer. It is contemplated that external device 40 maybe in wired or wireless connection to a communications network viatelemetry module 58 for transferring data to a remote database orcomputer to allow remote management of the patient 12.

FIG. 2 is a conceptual diagram of intracardiac pacemaker 100 shown inFIG. 1. Pacemaker 100 includes electrodes 162 and 164 spaced apart alongthe housing 150 of pacemaker 100 for sensing cardiac EGM signals anddelivering pacing pulses. Electrode 164 is shown as a tip electrodeextending from a distal end 102 of pacemaker 100, and electrode 162 isshown as a ring electrode along a mid-portion of housing 150, forexample adjacent proximal end 104. Distal end 102 is referred to as“distal” in that it is expected to be the leading end as it advancedthrough a delivery tool, such as a catheter, and placed against a targetpacing site.

Electrodes 162 and 164 form a cathode and anode pair for bipolar cardiacpacing and sensing. Electrodes 162 and 164 may be positioned on or asnear as possible to respective proximal and distal ends 104 and 102 toincrease the inter-electrode spacing between electrodes 162 and 164. Inalternative embodiments, pacemaker 100 may include two or more ringelectrodes, two tip electrodes, and/or other types of electrodes exposedalong pacemaker housing 150 for delivering electrical stimulation toheart 26 and for sensing EGM signals. Electrodes 162 and 164 may be,without limitation, titanium, platinum, iridium or alloys thereof andmay include a low polarizing coating, such as titanium nitride, iridiumoxide, ruthenium oxide, platinum black among others.

Electrodes 162 and 164 may be positioned at locations along pacemaker100 other than the locations shown or along an insulated conductorextending away from housing 150. For example, one or both of electrodes162 and/or 164 may be carried by a flexible insulated, electricalconductor extending away from housing 150 at proximal end 104 or distalend 102 for increasing the inter-electrode spacing between electrodes162 and 164. An intracardiac pacemaker having a flexible conductiveextender is generally disclosed in commonly-assigned, pre-grant U.S.Publication No. 2013/0035748 (Bonner, et al.), hereby incorporatedherein by reference in its entirety.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated. Inother examples, the entirety of the housing 150 may function as anelectrode instead of providing a localized electrode such as electrode162. Alternatively, electrode 162 may be electrically isolated from theother portions of the housing 150.

The housing 150 includes a control electronics subassembly 152, whichhouses the electronics for sensing cardiac signals, producing pacingpulses and controlling therapy delivery and other functions of pacemaker100. Housing 150 further includes a battery subassembly 160, whichprovides power to the control electronics subassembly 152. Batterysubassembly 160 may include features of the batteries disclosed incommonly-assigned U.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S.Pat. No. 8,541,131 (Lund, et al.), both of which are hereby incorporatedby reference herein in their entirety.

Pacemaker 100 may include a set of fixation tines 166 to securepacemaker 100 to or against cardiac tissue, e.g., by interacting withthe ventricular trabeculae. Fixation tines 166 are configured to anchorpacemaker 100 to position electrode 164 in operative proximity to atargeted tissue for delivering therapeutic electrical stimulationpulses. Numerous types of active and/or passive fixation members may beemployed for anchoring or stabilizing pacemaker 100 in an implantposition. Pacemaker 100 may include a set of fixation tines as disclosedin commonly-assigned, pre-grant publication U.S. 2012/0172892 (Grubac,et al.), hereby incorporated herein by reference in its entirety.

Pacemaker 100 may further include a delivery tool interface 158.Delivery tool interface 158 may be located at the proximal end 104 ofpacemaker 100 and is configured to connect to a delivery device, such asa catheter, used to position pacemaker 100 at an implant location duringan implantation procedure, for example within a heart chamber. A reducedsize of pacemaker 100 enables implantation wholly within a heartchamber. It is recognized that pacemaker 100 may be adapted in size,shape, electrode location or other physical characteristics according tothe heart chamber or location in which it will be implanted.

FIG. 3 is a functional block diagram of an example configuration ofpacemaker 100 shown in FIG. 2. Pacemaker 100 includes a pulse generator202, a sensing module 204, a control module 206, telemetry module 208,memory 210, activity sensor 212 and a power source 214. As used herein,the term “module” refers to an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsthat provide the described functionality. The particular form ofsoftware, hardware and/or firmware employed to implement thefunctionality disclosed herein will be determined primarily by theparticular system architecture employed in the IMD system 10 and by theparticular detection and therapy delivery methodologies employed by theIMD system 10. Providing software, hardware, and/or firmware toaccomplish the described functionality in the context of any modern IMDsystem, given the disclosure herein, is within the abilities of one ofskill in the art. The functions attributed to pacemaker 100 and ICD 14herein may be embodied as one or more processors, controllers, hardware,firmware, software, or any combination thereof. Depiction of differentfeatures as specific circuitry or modules is intended to highlightdifferent functional aspects and does not necessarily imply that suchfunctions must be realized by separate hardware or software componentsor by any particular architecture. Rather, functionality associated withone or more modules, processors, or circuits may be performed byseparate components, or integrated within common hardware, firmware orsoftware components. For example, pacing control operations performed bypacemaker 100 may be implemented in control module 206 executinginstructions stored in memory 210 and relying on input from sensingmodule 204.

Memory 210 may include computer-readable instructions that, whenexecuted by control module 206, cause control module 206 to performvarious functions attributed throughout this disclosure to pacemaker100. The computer-readable instructions may be encoded within memory210. Memory 210 may include any non-transitory, computer-readablestorage media including any volatile, non-volatile, magnetic, optical,or electrical media, such as a random access memory (RAM), read-onlymemory (ROM), non-volatile RAM (NVRAM), electrically-erasableprogrammable ROM (EEPROM), flash memory, or other digital media with thesole exception being a transitory propagating signal. Memory 210 storestiming intervals, counters, or other data used by control module 206 tocontrol the delivery of pacing pulses by pulse generator 202.

Pulse generator 202 generates electrical stimulation pulses that aredelivered to heart tissue via electrodes 162 and 164. Electrodes 162 and164 may be housing-based electrodes as shown in FIG. 2, but one or bothelectrodes 162 and 164 may alternatively be carried by an insulated,electrical conductor extending away from the pacemaker housing.

Pulse generator 202 may include one or more capacitors and a chargingcircuit to charge the capacitor(s) to a programmed pacing pulse voltage.At appropriate times, as controlled by a pace timing and control moduleincluded in control module 206, the capacitor is coupled to pacingelectrodes 162 and 164 to discharge the capacitor voltage and therebydeliver the pacing pulse. Pacing circuitry generally disclosed in theabove-incorporated U.S. Pat. No. 5,507,782 (Kieval, et al.) and incommonly assigned U.S. Pat. No. 8,532,785 (Crutchfield, et al.), both ofwhich patents are incorporated herein by reference in their entirety,may be implemented in pacemaker 100 for charging a pacing capacitor to apredetermined pacing pulse amplitude and delivering a pacing pulse underthe control of control module 206.

Control module 206 controls pulse generator 202 to deliver a pacingpulse in response to expiration of a pacing escape interval according toprogrammed therapy control parameters stored in memory 210. The pacetiming and control module included in control module 206 may include anescape interval timer or counter that is set to a pacing escape intervalused for controlling the timing of pacing pulses relative to a paced orsensed event. Upon expiration of a pacing escape interval, a pacingpulse is delivered. If a cardiac event is sensed during the pacingescape interval by sensing module 204, the scheduled pacing pulse may beinhibited, and the pacing escape interval may be reset to a new timeinterval.

Sensing module 204 receives cardiac EGM signals developed acrosselectrodes 162 and 164. A cardiac event may be sensed by sensing module204 when the EGM signal crosses a sensing threshold, which may be anauto-adjusting sensing threshold. In response to a sensing thresholdcrossing, sensing module 204 passes a sensed event signal to controlmodule 206 for use in controlling the timing of pacing pulses.

Pacemaker 100 may be configured to provide rate responsive cardiacpacing to meet the metabolic demand of the patient as the patienttransitions between different levels of patient activity, e.g., rest,moderate activity, and maximum exertion. Control module 206 may use asignal from activity sensor 212 for determining a patient-activity basedsensor-indicated rate (SIR) used to control the rate of pacing pulsedelivery. For example, an escape interval timer included in controlmodule 206 may be set to a pacing escape interval corresponding to aSIR, and the pacing escape interval may be adjusted as the SIR changesin response to the activity sensor signal.

Activity sensor 212 may be embodied as a piezoelectric accelerometer forproducing a signal correlated to patient body motion. The use of anaccelerometer in an intracardiac device for obtaining a patient activitysignal is generally disclosed in U.S. Pat. Publication No. 2015/0217119filed on Feb. 6, 2014 (Nikolski, et al.), incorporated herein byreference in its entirety. The use of a patient activity signal forproviding rate-responsive pacing is generally disclosed in U.S. Pat. No.7,031,772 (Condie, et al.), incorporated herein by reference in itsentirety.

Control module 206 receives an activity signal from activity sensor 212and may be configured to determine an activity metric from the signal ata desired frequency for use in determining a sensor-indicated pacingrate. The SIR may vary between a programmed lower rate (LR) duringperiods of rest and a programmed maximum upper pacing rate duringperiods of maximum exertion. The SIR may be controlled according to aSIR transfer function as described below which may include differentrates of change of the SIR over different ranges of the activity metric.

In some examples, the activity metric is determined by control module206 as an activity count. In these instances, control module 206includes a counter to track the activity count as the number of timesthe signal from activity sensor 212 crosses a threshold during anactivity count interval, for example a 2-second interval. The count atthe end of each activity count interval is correlated to motion imposedon the sensor during the activity count interval, including patient bodymotion, and can therefore be correlated to patient activity andmetabolic demand. In some examples, the activity counts determined overa monitoring interval are used by control module 206 for determining anactivity count below which the SIR will remain at the programmed LR.This activity count is referred to herein as the “LR set point.”Activity counts below the LR set point indicate a resting state of thepatient that does not require pacing above the programmed LR. Controlmodule 206 will determine a SIR that is greater than the programmed LRwhen the activity count is greater than the LR set point, according to aSIR transfer function.

In other examples, an activity metric may be obtained from the activitysensor signal by integrating or summing activity signal sample pointsover an activity count interval, e.g., a two second interval thoughlonger or shorter intervals of time may be used for determining anactivity metric. In various examples, the activity metrics accumulatedover a given time interval are used to determine the LR set point.Methods for establishing and updating a LR set point are generallydisclosed in U.S. Patent Publication No. 2016/0144191, filed Nov. 25,2014 (Demmer, et al.), incorporated herein by reference in its entirety.

As described below, control module 206 is configured to confirm ahemodynamically unstable rhythm based at least in part on a signal fromactivity sensor 212. The techniques described herein are applicable toan activity sensor that is sensitive heart motion such that an activitymetric determined when the patient is still and at rest can have anon-zero value due to heart motion.

A LR set point is an activity metric value that is established bycontrol module 206 to filter baseline heart motion signals from activitymetrics that may be used to set a rate responsive pacing rate to meetthe metabolic demand of the patient. Activity metrics less than the LRset point but greater than an unstable rhythm threshold represent anormal range of activity metrics obtained when the patient is in aresting state but the activity sensor signal includes heart motionduring a hemodynamically stable heart rhythm. The unstable rhythmdetection threshold may be defined as an activity metric value that isless than the LR set point. Activity metrics less than the unstablerhythm detection threshold indicate that the heart motion normallycontributing to the activity sensor signal has diminished significantly.An activity metric that is less than the unstable rhythm detectionthreshold indicates a hemodynamically unstable rhythm and therefore arhythm that warrants immediate shock treatment. In some examples, theunstable rhythm detection threshold may be based on the LR set pointthat is determined to filter heart motion from activity metrics used fordetermining a SIR and setting a rate responsive pacing rate.

The pacemaker 100 may be configured to deliver ATP in response todetecting VT if unstable rhythm detection criteria based on the activitysensor signal are not satisfied. If the unstable rhythm detectioncriteria based on the activity sensor signal are met, the pacemaker 100does not deliver ATP. ICD 14 detects the shockable rhythm usingECG-based VT detection criteria and delivers an appropriate shocktherapy.

Other types of activity sensors other than an accelerometer may producea signal correlated to the patient metabolic demand. For example,sensors of respiratory activity, such as minute ventilation, blood ortissue oxygen saturation, or another sensor of the patient's body motionor physical activity may be used for providing control module 206 with asignal correlated to metabolic demand for controlling a rate responsivetherapy. Various examples of other types of implantable sensors that maybe implemented with a rate responsive pacemaker for controlling pacingrate based on metabolic demand are generally described in U.S. Pat. No.5,755,740 (Nappholz), U.S. Pat. No. 5,507,785 (Deno), and U.S. Pat. No.5,312,454 (Roline). The techniques disclosed herein may be implementedin conjunction with any type of activity sensor that produces a signalthat indicates a non-zero activity level due to heart motion during aresting state, i.e., in the absence of actual physical activity orexertion by the patient.

Pacemaker 100 may further include one or more other physiologicalsensors for monitoring the patient, such as a pressure sensor, anacoustical sensor, an oxygen sensor, or any other implantablephysiological sensor. In some cases, activity sensor 212 may beimplemented as a three-dimensional accelerometer used for determiningpatient activity along any one of the three dimensions and used fordetecting changes in patient body posture using the three-dimensionalaccelerometer signal. A multi-dimensional accelerometer for detectingpatient posture changes is generally disclosed in U.S. Pat. No.5,593,431 (Sheldon), hereby incorporated herein by reference in itsentirety.

Power source 214 provides power to each of the other modules andcomponents of pacemaker 100 as required. Control module 206 may executepower control operations to control when various components or modulesare powered to perform various pacemaker functions. Power source 214 mayinclude one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 214 and other pacemaker modules and components are notshown in FIG. 3 for the sake of clarity.

Telemetry module 208 includes a transceiver and associated antenna fortransferring and receiving data from external device 40 via a radiofrequency (RF) communication link as described above. Pacemaker 100 mayreceive pacing and sensing control parameters via telemetry module 208and store the control parameter values in memory 210 for access bycontrol module 206.

FIG. 4 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 includes software,firmware and hardware that cooperatively monitor one or more ECGsignals, determine when a CV/DF shock is necessary, and deliverprescribed CV/DF therapies. In some examples, ICD 14 may be coupled to alead, such as lead 16, carrying electrodes, such as electrodes 24, 28and 30, positioned in operative relation to the patient's heart fordelivering cardiac pacing pulses, e.g., post-shock bradycardia pacing,in addition to shock therapies.

ICD 14 includes processor and control module 80, memory 82, therapydelivery module 84, electrical sensing module 86, telemetry module 88,and cardiac signal analyzer 90. A power source 98 provides power to thecircuitry of ICD 14, including each of the modules 80, 82, 84, 86, 88,and 90 as needed. Power source 98 may include one or more energy storagedevices, such as one or more rechargeable or non-rechargeable batteries.

The functional blocks shown in FIG. 3 represent functionality that maybe included in ICD 14 and may include any discrete and/or integratedelectronic circuit components that implement analog and/or digitalcircuits capable of producing the functions attributed to ICD 14 herein.For example, the modules may include analog circuits, e.g.,amplification circuits, filtering circuits, and/or other signalconditioning circuits. The modules may also include digital circuits,e.g., analog-to-digital converters, combinational or sequential logiccircuits, integrated circuits, processors, ASICs, memory devices, etc.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device.

Furthermore, memory 82 may include non-transitory computer readablemedia storing instructions that, when executed by one or more processingcircuits, cause processor and control module 80 or other ICD modules toperform various functions attributed to ICD 14. The non-transitorycomputer readable media storing the instructions may include any of themedia listed above, with the sole exception being a transitorypropagating signal. The functions attributed to IMD system 10 may beexecuted by system control circuitry including ICD processor and controlmodule 80 and pacemaker control module 206. The system control circuitrymay execute instructions stored by discrete or distributednon-transitory, computer-readable storage media to cause IMD system 10to perform the functions disclosed herein. Processor and control module80 communicates with therapy delivery module 84, cardiac signal analyzer90 and electrical sensing module 86 for sensing cardiac electricalactivity, detecting cardiac rhythms, and generating electricalstimulation therapies in response to sensed signals. Therapy deliverymodule 84 and electrical sensing module 86 are electrically coupled toelectrodes 24, 28, and 30 carried by lead 16 (shown in FIG. 1) andhousing 15, which may serve as a common or ground electrode.

Electrical sensing module 86 is selectively coupled to sensingelectrodes 28, 30 and housing 15 in order to monitor electrical activityof the patient's heart. Electrical sensing module 86 may additionally beselectively coupled to defibrillation electrode 24. Sensing module 86 isenabled to selectively monitor one or more sensing vectors selected fromthe available electrodes 24, 28, 30 and 15. For example, sensing module86 may include switching circuitry for selecting which of electrodes 24,28, 30 and housing 15 are coupled to sense amplifiers included insensing module 86. Switching circuitry may include a switch array,switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple sense amplifiers to selected electrodes.

In some examples, electrical sensing module 86 includes multiple sensingchannels for sensing multiple ECG sensing vectors selected fromelectrodes 24, 28, 30 and housing 15. For example, sensing module 86 mayinclude two sensing channels. Each sensing channel may include a senseamplifier or other cardiac event detection circuitry for sensing cardiacevents, e.g., R-waves, from the received ECG signal developed across theselected electrodes 24, 28, 30 or 15. Each time the received ECG signalcrosses the auto-adjusting sensing threshold outside a blankinginterval, a cardiac event sense signal, such as an R-wave sense eventsignal, is produced and passed to processor and control module 80 and/orcardiac signal analyzer 90 for use in detecting VT.

Sensing module 86 may include an analog-to-digital converter forproviding a digital ECG signal from one or all available sensingchannels to processor and control module 80 and/or cardiac signalanalyzer 90. For example two ECG signals as described above may each beconverted to a multi-bit digital signal by sensing module 86 andprovided to tachyarrhythmia detector 94 for performing ECG morphologyanalysis. Analysis of the ECG signal morphology may be performed bycardiac signal analyzer 90 for detecting VT.

In some examples, sensing module 86 is configured to detect pacingpulses delivered to heart 26 by pacemaker 100. For example, ATP may bedelivered by pacemaker 100 in response to pacemaker 100 detecting VT.ICD 14 is configured to withhold or delay a shock therapy when ashockable, VT rhythm is detected by cardiac signal analyzer 90 and ATPpulses are being sensed by sensing module 86. In this way, pacemaker 100may deliver ATP therapy to terminate the VT precluding the need for ashock. As such, ICD sensing module 86 may include a sensing channelconfigured with filtering and sensing threshold properties to enablesensing of a pacing pulse to enable ICD 14 to detect ATP pulses beingdelivered by pacemaker 100. Pacing pulse sensing may correspond toapparatus and techniques generally disclosed in U.S. Patent PublicationNo. 2015/0305642 and in U.S. Patent Publication No. 2015/0305640, bothfiled on Apr. 25, 2014 and incorporated herein by reference in theirentirety.

ATP may be detected by cardiac signal analyzer 90 when pacing pulsesense event signals are received from sensing module 86 at a rate orinterval(s) that meet an expected ATP therapy rate (e.g., versus abradycardia pacing rate) and/or verifying that the rate of pacing pulsesense event signals are received at a rate that is faster than theimmediately preceding detected ventricular rate.

ATP delivered by pacemaker 100 may include a burst of ATP pulses, e.g.6-16 pacing pulses delivered at intervals that are less than 90% of thedetected VT cycle length. Other examples of an ATP therapy delivered bypacemaker 100 may include Ramp ATP that includes a series ofprogressively faster ATP pulses or Burst+Ramp that includes a fixed rateburst of ATP pulses followed by one or more pulses that are delivered atprogressively shorter intervals than the preceding pacing pulseinterval. For instance, each successive interval between ATP pacingpulses may be 20 ms shorter than the previous interval during Ramp ATP.Accordingly, ICD 14 may detect ATP when pacing pulses are sensed bysensing module 86 at intervals that are expected during an ATP therapy.

To illustrate, if VT is detected by ICD 14 with a cycle length (RRinterval) of 400 ms, ICD 14 may detect ATP delivered by pacemaker 100 inresponse to sensing a required number of pacing pulses, e.g., at leastthree pacing pulses, at intervals that are less than 360 ms. A typicalATP therapy delivered by pacemaker 100 in response to detecting VT withthe cycle length of 400 ms, and not confirming a hemodynamicallyunstable rhythm, might be a burst of 8 pacing pulses at 350 ms. As such,ICD 14 will detect the ATP based on sensing pacing pulses at intervalsless than 360 ms.

As described below, pacemaker 100 is configured to confirm ahemodynamically unstable rhythm based on the activity sensor signal. Ifa hemodynamically unstable rhythm is not confirmed, pacemaker 100delivers ATP, which is detected by ICD 14 causing ICD 14 to withhold ordelay a shock. If the VT is redetected after ATP and ATP pulses are nolonger detected by ICD 14, processor and control module 80 may controltherapy delivery module 84 to deliver the previously withheld or delayedshock.

Cardiac signal analyzer 90 includes a tachyarrhythmia detector 94 fordetecting VT and discriminating VT from non-shockable rhythms. Cardiacsignal analyzer 90 may further include a timing circuit 92 that includesvarious timers and/or counters for measuring time intervals, such as RRintervals, setting time windows such as morphology template windows,morphology analysis windows or for performing other timing relatedfunctions of cardiac signal analyzer 90 including synchronizingcardioversion shocks or other therapies delivered by therapy deliverymodule 84 with sensed cardiac events.

In some examples, the timing of R-wave sense event signals received fromsensing module 86 is used by timing circuit 94 to determine RR intervalsbetween sense event signals. Tachyarrhythmia detector 94 may count RRintervals measured by timing circuit 92 that fall into different ratedetection zones for determining a ventricular rate or performing otherrate- or interval-based assessment for detecting VT and discriminatingVT and non-shockable rhythms.

Examples of algorithms that may be performed by ICD 14 for detecting,discriminating and treating shockable rhythms, which may be adapted toinclude techniques described herein for controlling shock therapydelivery, are generally disclosed in U.S. Pat. No. 5,354,316 (Keimel);U.S. Pat. No. 5,545,186 (Olson, et al.); U.S. Pat. No. 6,393,316(Gillberg et al.); U.S. Pat. No. 7,031,771 (Brown, et al.); U.S. Pat.No. 8,160,684 (Ghanem, et al.), and U.S. Pat. No. 8,437,842 (Zhang, etal.), all of which patents are incorporated herein by reference in theirentirety. The detection algorithms are highly sensitive and specific forthe presence or absence of life threatening, shockable VT.

Therapy delivery module 84 includes a high voltage (HV) therapy deliverymodule including one or more HV output capacitors and, in someinstances, a low voltage therapy delivery module. When a shockable VTrhythm is detected the HV capacitors are charged to a pre-programmedvoltage level by a HV charging circuit. Control module 80 applies asignal to trigger discharge of the HV capacitors upon detecting afeedback signal from therapy delivery module 84 that the HV capacitorshave reached the voltage required to deliver a programmed shock energy.In this way, control module 80 controls operation of the high voltageoutput circuit of therapy delivery module 84 to deliver high energycardioversion/defibrillation shocks using defibrillation electrode 24and housing 15. Timing circuit 92 may be used to control R-wavesynchronized shock pulses delivered by therapy delivery module 84.

It should be noted that implemented arrhythmia detection algorithms mayutilize not only ECG signal analysis methods but may also utilizesupplemental sensors 96, such as blood pressure, tissue oxygenation,respiration, patient activity, heart sounds, and the like, forcontributing to a decision by processing and control module 80 to applyor withhold a therapy.

User-programmable therapy delivery control parameters may be programmedinto memory 82 via telemetry module 88. Telemetry module 88 includes atransceiver and antenna for communicating with external device 40 (shownin FIG. 1) using RF communication. Telemetry module 88 may receivedownlink telemetry from and send uplink telemetry to external device 40.

ECG episode data related to the detection of VT and the delivery of acardioversion or defibrillation shock may be stored in memory 82 andtransmitted by telemetry module 88 to external device 40 upon receipt ofan interrogation command. Clinician review of episode data facilitatesdiagnosis and prognosis of the patient's cardiac state and therapymanagement decisions, including selecting programmable controlparameters used for detecting shockable VT rhythms and deliveringtherapy.

In some examples, ICD 14 and pacemaker 100 may be configured tocommunicate with each other via wireless RF telemetry signalstransmitted and received by ICD telemetry module 88 and pacemakertelemetry module 208. In some examples, pacemaker 100 may be configuredto transmit an RF signal to ICD 14 to cause ICD 14 to withhold or delaya shock therapy until pacemaker 100 has attempted to terminate adetected tachyarrhythmia by delivering ATP. In other instances,pacemaker 100 may transmit an RF signal to ICD 14 to cause ICD 14 toproceed immediately with a shock therapy without waiting for pacemaker100 to delivery an ATP therapy.

FIG. 5 is a plot 300 of a sensor-indicated rate (SIR) transfer functionthat may be used by pacemaker 100 for controlling therapies delivered toheart 26 according to one example. Activity sensor 212 may be anaccelerometer that produces a signal correlated to patient body motion.Activity metrics may be determined by pacemaker control module 206 fromthe accelerometer signal as a count of the total number of thresholdcrossings of the accelerometer signal or an integrated value of theaccelerometer signal over a predetermined n-second interval, e.g., a2-second interval. The SIR transfer function shown in FIG. 5 may beestablished using activity counts determined from the accelerometersignal that reflect a patient's activity profile over a typical day orweek, user-programmed parameters or a combination of both.

In plot 300, SIR is plotted along the y-axis 302 as a function ofactivity count plotted along the x-axis 304. Pacemaker control module206 may establish a lower rate (LR) set point 308 based on an analysisof the activity counts determined over an interval of time. Whenpacemaker 100 is enabled to provide rate-responsive pacing based onactivity counts determined from the activity sensor signal, the pacingrate is not adjusted above a lower rate 306, sometimes referred to asthe “base pacing rate,” as long as the activity counts are at or belowthe LR set point 308.

As the activity count increases above the LR set point 308, the SIR maybe determined according to the established transfer function between theSIR and the activity count. For example, an activity of daily living(ADL) lower set point 312 and ADL upper set point 318 may be establishedas the lower and upper boundaries of an activity count range that isexpected to encompass the patient's activity level during normal dailyactivities and moderate activity, such as moving about the house,driving a car, daily chores, etc. The SIR may be increased from thelower pacing rate 306 to the ADL rate 314 according to a slope 310between the LR set point 308 and the ADL lower set point 312. The SIRremains at the ADL rate 314 over the ADL range 316 between the ADL lowerset point 312 and the ADL upper set point 318. An activity count abovethe upper ADL set point 318 will cause the pacemaker 100 to adjust theSIR according to a second slope 320 as a function of activity count upto a maximum upper rate set point 322. The SIR is set to the maximumupper pacing rate 324 for all activity counts greater than the maximumupper rate set point 322. Each of the lower ADL set point 312, upper ADLset point 318 and maximum upper rate set point 322 may be tailored to apatient's particular needs based on the patient's activity counthistory.

The LR set point 308 and an unstable rhythm threshold 350 less than theLR set point 308 may be established by the pacemaker 100 based on ananalysis of activity counts sampled over an adjustment interval in someexamples. For instance, an activity count may be determined every twoseconds over a 24-hour adjustment interval. The activity count value ata predetermined percentile 345 of all activity count values accumulatedover the adjustment interval is selected as the LR set point 308 in oneexample. The predetermined percentile 345 may be established as thepercentage of time the patient is expected to require pacing at the LR306, which can also be thought of as the percentage of time that thepatient is expected to be at rest or non-active. The activity counts ina resting range 346 extending from an activity count of 0 up to thepreviously established percentile 345 represent activity counts that mayoccur when the patient is at rest, e.g., sleeping, napping, sitting orotherwise inactive, and not requiring a pacing rate greater than thelower pacing rate to meet metabolic demand. The activity counts belowpercentile 345 are highly likely to be due primarily to heart motion anddo not represent physical activity of the patient. The activity countvalues in a non-resting activity range 348 extending from the previouslyestablished percentile 345 to a maximum possible activity countrepresent activity counts that are expected to occur when the patient isactive (not resting) and requires a pacing rate greater than the LR 306to meet the patient's metabolic demand. To illustrate, the percentile345 may be selected as 85% such that the SIR is at the LR 306approximately 85% of the time and will be increased above the LR 306approximately 15% of the time.

The analysis of the activity counts over an adjustment interval may bethought of in terms of a frequency plot 340. The number of activitycounts occurring during a predefined time interval is shown along they-axis 342 for each activity count value shown along the x-axis 344. Inone example, the range of possible activity count values may be dividedinto predetermined activity count bins. The activity counts occurring ineach bin are counted over the predefined time interval. The activitycount bin at the predetermined patient activity percentile 345 isidentified and set as the LR set point 308.

The LR set point 308 may be increased or decreased over time based onaccumulated activity counts in order to maintain the number of activitycounts that are greater than the LR set point 308 within a range of anexpected number of activity counts greater than the LR set point.Continuing with the example given above, if the patient activitypercentile 345 is 85%, the activity counts in range 348 greater than theLR set point 308 are expected to be approximately 15% of all theactivity counts determined over a given time interval. If more than 15%of the activity counts are greater than the LR set point 308 during apredetermined time interval, the LR set point 308 may be increased. Ifless than 15% of the activity counts are greater than the LR set point308, the LR set point 308 may be decreased by pacemaker control module206. A pacemaker and methods for establishing a LR set point aregenerally disclosed in the above-incorporated U.S. Patent PublicationNo. 2016/0144191 filed Nov. 25, 2014.

In addition to establishing LR set point 308, pacemaker control module206 may establish an unstable rhythm detection threshold 350 forconfirming a hemodynamically unstable rhythm based on an analysis ofactivity counts. The unstable rhythm detection threshold 350 is lessthan the LR set point 308 and may be defined as a percentile of allactivity counts determined over a given time interval, e.g., the firstpercentile, fifth percentile or tenth percentile of all activity counts.Alternatively, the unstable rhythm detection threshold 350 may bedefined as an activity count that is a percentage of the LR set point308, e.g., 10% or 20% of the LR set point 308. A normal resting range352 of activity count values occurring between the LR set point 308 andthe established unstable rhythm detection threshold 350 represents therange of a vast majority, e.g., 90% or more, of the activity countvalues that are expected to occur when the patient is in a resting stateand the activity count value is due primarily to heart motion. Anactivity count less than the normal resting range 352 indicates asignificant decrease in heart motion that is evidence for confirming ahemodynamically unstable rhythm.

An activity count that is less than the unstable rhythm detectionthreshold 350, therefore, indicates that even heart motion that normallycontributes to the activity sensor signal has diminished significantly.If a fast ventricular rate has been detected by the pacemaker controlmodule 206, the fast rate of the electrical cardiac signals is highlylikely to be a hemodynamically unstable rhythm that requires a shocktherapy if the activity count is less than the unstable rhythm detectionthreshold 350, i.e., less than the normal resting range 352 of activitycounts.

FIG. 6 is a flow chart 400 of a process performed by IMD system 10 shownin FIG. 1 according to one example. At block 402, the pacemaker 100detects a fast ventricular rate from the EGM signal received bypacemaker 100. The fast ventricular rate may be detected according totachyarrhythmia detection criteria of an implemented detection algorithmin pacemaker 100. ICD 14 is expected to simultaneously be detecting afast ventricular rate from the ECG signal received by ICD 14, based on atachyarrhythmia detection algorithm implemented in ICD 14. The detectionalgorithms implemented by pacemaker 100 and ICD 14 may differ. Theimplemented algorithms may include determining RR intervals betweenconsecutively sensed R-waves and counting RR intervals that fall into VTinterval ranges to determine if a required number of VT intervals havebeen reached to detect VT. Detecting a fast ventricular rate mayadditionally or alternatively include analyzing R-wave morphology,determining a gross signal morphology, or other analysis of the EGMsignal received by the pacemaker 100 or the ECG signal received by ICD14.

In response to detecting a fast ventricular rate, the pacemaker 100 isconfigured to confirm that the fast rate is a hemodynamically unstablerhythm based on the activity sensor signal. In some examples, thepacemaker control module 206 may first determine if the activity sensorsignal quality is reliable for use in confirming a hemodynamicallyunstable rhythm to support a therapy delivery decision.

At block 404, pacemaker control module 206 may determine a signalquality metric of the activity sensor signal. In one example, activitycounts are determined from the activity sensor signal every two seconds.A signal quality metric may be a determination of the distribution ofthe activity counts over a preceding time interval to verify that thedistribution meets an expected distribution. For example, the pacemakercontrol module 206 may determine a percentage of activity counts over apreceding time interval (e.g., one or more minutes, one hour, eighthours, or up to 24 hours prior to the onset of the fast ventricularrate) that are greater than the LR set point and/or less than the LR setpoint. In another example, the pacemaker control module 206 maydetermine a percentage of the activity counts accumulated over apreceding time interval prior to the onset of the fast ventricular ratethat are less than the shock detection threshold.

In yet another example, the pacemaker control module 206 may compare acurrent distribution of activity counts less than the LR set point 308to an expected or previous distribution of activity counts less than theLR set point 308. An example distribution of the activity counts lessthan the LR set point 308 is shown in FIG. 5. If the distribution ofactivity counts less than the LR set point 308 changes significantlycompared to a previous distribution, the activity sensor signal may notbe reliable for confirming a hemodynamically unstable rhythm. To detecta significant change in the activity count distribution in the restingrange 346, the pacemaker control module 206 may determine a mean,median, mode, standard deviation, total range, lower quartile range,upper quartile range, or any combination thereof and/or otherindicator(s) of the distribution of activity counts less than the LR setpoint 308 and compare the determined indicator(s) to a previous restingrange activity count distribution.

Additionally or alternatively, the raw activity sensor signal may beanalyzed for signal peaks occurring at regular time intervals asevidence of heart motion during ventricular systole occurring at aregular heart rate prior to the onset of the fast ventricular rate.Regular heart motion signals prior to the onset of the fast ventricularrate would be evidence of a reliable activity sensor signal.

The signal quality metric is compared to reliability criteria at block406 to determine if the activity sensor signal is reliable forconfirming a hemodynamically unstable rhythm. For example, if more thanan expected percentage of activity counts are less than the unstablerhythm detection threshold, less than the LR set point, and/or thedistribution over the resting range 346 has changed during a precedingtime interval prior to the onset of the fast ventricular rate, theactivity sensor signal may be deemed unreliable for confirming ahemodynamically unstable rhythm. Additionally or alternative, thepercentage of activity counts greater than the LR set point may becompared to an expected target percentage at block 406 for determiningif the activity sensor signal is reliable. If the percentage of activitycounts greater than the LR set point fall outside the expectedpercentage range, the activity sensor signal may be determined to beunreliable.

Various activity sensor signal quality metrics may be determined andcompared to signal reliability criteria for determining if the activitysensor signal is reliable for use in discriminating between ahemodynamically unstable and hemodynamically stable rhythm. Generally,the determination made at block 410 is whether the activity metricsaccumulated over a preceding time period are within an expecteddistribution or represent an unexpected distribution of activity metricswhen a tachyarrhythmia is not being detected, e.g., during a normalsinus or paced rhythm. An unexpected distribution indicates that theactivity sensor may be functioning unreliably or the sensor signal hasbecome corrupted.

If the activity sensor signal is deemed unreliable based on anunexpected distribution of activity counts over a preceding timeinterval or other reliability criteria, a therapy decision is made atblock 408 by the pacemaker control module 206 without the use of theactivity sensor signal. The decision to deliver ATP or not is based onthe EGM signal analysis only (or in combination with signals from othersensors that may be available other than the activity sensor 212). Insome examples, if the activity sensor signal is considered unreliablefor confirming a hemodynamically unstable rhythm, pacemaker 100withholds therapy, and therapy delivery is controlled by ICD 14. Atblock 418, ICD 418 will not detect ATP delivery by pacemaker 100 andwill proceed with shock delivery at block 420 if the ICD 14 is detectingVT based on the VT detection algorithm implemented in ICD 14.

If the activity sensor signal is determined to be reliable at block 406,the pacemaker control module 206 determines the activity metric from theactivity sensor signal at block 410 in response to detecting the fastventricular rate. The activity metric may be determined at theexpiration of each one or more time intervals after the fast rate isdetected. Additionally or alternatively, one or more activity metricvalues determined immediately prior to the criteria being met fordetecting a fast ventricular rate may be used for confirming the fastrhythm is a hemodynamically unstable rhythm.

The one or more activity metrics are analyzed at block 412 to determineif unstable rhythm criteria are met. For example, at least one activitymetric is compared to an unstable rhythm detection threshold at block412. An activity metric less than the unstable rhythm detectionthreshold, e.g., less than a normal resting activity metric range 352,is evidence of a hemodynamically unstable rhythm. One or more activitymetrics less than the unstable rhythm detection threshold may berequired to confirm a hemodynamically unstable rhythm. Examples ofmethods for determining if unstable rhythm criteria are met at block 412are described below in conjunction with FIGS. 7 and 8. If the unstablerhythm criteria are not met, the fast ventricular rate is detected as ahemodynamically stable VT, and ATP may be delivered at block 414. Beforedelivering ATP, pacemaker control module 206 may be configured to verifythat a shock, delivered by the ICD 14, has not been sensed by thepacemaker sensing module 204 after the fast ventricular rate wasdetected or during a predefined preceding time interval.

In one example, at least two activity metrics are determined at block410 as activity counts determined at two consecutive 2-second intervals.The two activity counts may be the most recent two activity countsdetermined after the onset of the fast ventricular rate but prior to VTdetection criteria being met. In other cases, one activity count afterthe onset of the fast ventricular rate and prior to EGM-based VTdetection criteria being met and one activity count after VT detectionare compared to the unstable rhythm criteria. In one example, if atleast one of the two activity counts is less than the unstable rhythmthreshold 350 (FIG. 5), the rhythm is confirmed to be a hemodynamicallyunstable rhythm. In another example, both of the two consecutiveactivity counts must be less than the unstable rhythm threshold 350 inorder to confirm a hemodynamically unstable rhythm. In other examples,more than two activity counts are used for confirming thehemodynamically unstable rhythm.

If a hemodynamically unstable rhythm is confirmed, ATP is withheld bythe pacemaker 100. The ICD 14 does not detect ATP at block 418 anddelivers a CV/DF shock at block 420 as long as other shockable rhythmdetection criteria used by the ICD 14 have been met.

If the activity metrics determined at block 410 do not meet unstablerhythm criteria at block 412, the pacemaker control module 206 controlspulse generator 202 to deliver ATP at block 414. In one example, ICDcardiac signal analyzer 90 and/or processor and control module 80 is/areconfigured to detect the ATP pulses at block 418 based on a rate orexpected pattern of pacing pulse sense event signals received from ICDsensing module 86. In response to detecting ATP pulses, the ICDprocessor and control module 80 controls the therapy delivery module 84to withhold shock therapy at block 422 until ATP pulses are no longerdetected (block 424) and a shockable, VT rhythm is still being detected(block 426). If ATP pulses are no longer detected, and a shockable, VTrhythm is still being detected by ICD 14, the shock is delivered atblock 420. If ATP pulses are no longer detected and VT is no longerdetected, (ATP was successful in terminating the VT), a scheduled shockis cancelled at block 428.

In some cases, more than one ATP attempt to terminate the detected VTmay be made by pacemaker 100 at block 414. Accordingly, the ICD 14 maydetect more than one ATP sequence at blocks 418 and 424 and delay ascheduled shock until ATP is no longer detected. In other examples, thepacemaker 100 may be configured to deliver a single ATP sequence, andICD cardiac signal analyzer 90 may be configured to re-detect theshockable, VT rhythm and deliver a delayed shock at block 420 after apredetermined shock delay interval after ATP detection if the shockablerhythm is still being detected (block 424). ICD 14 may be configured towait for a maximum time interval for delivering a delayed shock. Themaximum time interval may correspond to a maximum number of attempts ofATP.

FIG. 7 is a timing diagram 500 illustrating the operations of ICD 14 andpacemaker 100 according to one example. Pacemaker control module 206determines activity counts 502, 510, 512, 514, 516 (or another activitymetric) at regular activity count sampling intervals 505, e.g., every 2seconds. A LR set point 504 and an unstable rhythm detection threshold506 are shown by dashed lines. The LR set point 504 and the unstablerhythm detection threshold 506 may be established by pacemaker 100 basedon an analysis of activity counts accumulated over a monitoring timeperiod as described above. Rate-responsive pacing may or may not beenabled in pacemaker 100. The LR set point 504 and unstable rhythmdetection threshold 506 are determined as the range of activity countsexpected to occur as a result of normal heart motion, when the patientis not exercising. Activity counts less than the unstable rhythmdetection threshold 506 are indicative of a hemodynamically unstablerhythm. If rate responsive pacing is enabled, activity counts at or lessthan the LR set point 504 are used to set the pacing rate to theprogrammed lower rate and activity counts greater than the LR set point504 are used to increase the pacing rate to a rate faster than theprogrammed lower rate according to a SIR transfer function, e.g., thetransfer function shown in FIG. 5.

Activity counts in the range between LR set point 504 and unstablerhythm threshold 506 represent counts due to heart motion, and activitycounts less than the unstable rhythm threshold 506 represent countsexpected when heart motion is significantly altered or diminished due toa hemodynamically unstable tachyarrhythmia. Unstable rhythm detectionthreshold 506 is less than the LR set point 504, at which the patient isconsidered to be at rest or non-active, and may be set based on LR setpoint 504. As such, the unstable rhythm detection threshold 506 is notmerely an indication of a patient's resting condition or inactive statesince the patient's normal resting condition activity count range existsprimarily between LR set point 504 and unstable rhythm detectionthreshold 506. Unstable rhythm detection threshold 506 represents adecrease from an activity count during normal heart motion at rest to anactivity count less than the normal resting range 352 (FIG. 5),indicative of a significant decrease in overall heart motion due to ahemodynamically unstable rhythm.

Pacemaker 100 senses ventricular events 520 and is configured to detectVT based on an analysis of the EGM signal(s) received by sensing module204. In the example of FIG. 7, a series of ventricular sensed events 522meet RR interval-based VT detection criteria resulting in VT detection524. In some cases, if at least eight out of twelve RR intervals areless than a VT detection interval, e.g., less than 300 ms, VT isdetected. In other examples, up to 24 RR intervals or more may berequired for making a VT detection, e.g., 16 out 24 RR intervals may berequired to be VT intervals, depending on the on the particular VTdetection criteria implemented in pacemaker 100 and programmed detectionparameters.

Upon detecting VT 524, pacemaker 100 analyzes one or more activitycounts to confirm a hemodynamically unstable rhythm. In one example, atleast the most recent activity count 512 preceding VT detection 524 iscompared to the unstable rhythm detection threshold 506. In anotherexample, at least the first or earliest activity count 514 immediatelyafter VT detection 524 is compared to the unstable rhythm detectionthreshold 506. One or more activity counts 510, 512 preceding the VTdetection 524 after the onset of the fast VS events 522 and/or one ormore activity counts 514 after the VT detection 524 may be used inconfirming a hemodynamically unstable rhythm.

The number of activity counts or other activity metrics used to confirma hemodynamically unstable rhythm in response to EGM-based VT detection524 may depend at least in part on the sampling interval 505 betweenactivity counts and the time required to make the EGM-based VT detection524 after onset of the fast rate of VS events 522. In the illustrativeexample given above, activity counts 510, 512 and 514 are determined attwo second intervals. If VT detection criteria requires at least 8 outof 12 consecutive RR intervals are less than a VT detection interval of300 ms or less, one or two activity counts 510 and 512 may occur duringthe eight to twelve RR intervals during the series of fast VS events 522leading up to VT detection 524. In other cases, when a greater number ofRR intervals are required make a VT detection, e.g., if at least 16 outof 24 RR intervals are required to fall in a VT interval zone, three ormore activity counts may occur during the series of VS events 522 thatlead to VT detection criteria being satisfied. All of the activitycounts determined after onset of a fast rate, i.e., the onset 525 of VSevents 522 that satisfy VT detection criteria, may be compared to theunstable rhythm threshold 506. A predetermined percentage of activitycounts determined after the onset 525 of the fast rate may be requiredto be less than the unstable rhythm threshold.

In the examples of FIG. 7 and FIG. 8 below, the activity counts aredetermined at regular sampling intervals 505 before and during VTdetection. In other examples, activity metrics may be determined moreoften, i.e., at shorter sampling intervals, to obtain more activitymetrics for confirming a hemodynamically unstable rhythm. The activitymetrics determined more frequently may be compared to criteriaappropriately adjusted for change in sampling rate. For example, ifactivity counts are determined over 2-second intervals when a fast rateis not being detected, activity counts may be determined every onesecond when a fast rate is being detected. The unstable rhythm threshold506 may be set taking into account the shorter activity count interval.

In one example, if at least one of the two most recent activity counts510 and 512 is less than the unstable rhythm detection threshold 506,the VT detection 524 is confirmed as a hemodynamically unstable rhythm.In another example, if both of the two most recent activity counts 510and 512 are equal to or less than the LR set point 504 and at least oneis less than the unstable rhythm detection threshold 506, ahemodynamically unstable rhythm is confirmed.

In another example, at least two consecutive activity counts arerequired to be less than the unstable rhythm detection threshold 506 inorder to confirm the VT detection 524 as a hemodynamically unstablerhythm. The two consecutive activity counts may both be before VTdetection 524, e.g., activity counts 510 and 512 during VS events 522,or one may be before VT detection 524 and one after, e.g., activitycounts 512 and 514. In some examples, pacemaker control module 206 mayidentify the activity counts 512 and 514 occurring closest in time tothe VT detection 524 and if both activity counts 512 and 514 are lessthan the unstable rhythm detection threshold 506, the VT detection 524is confirmed as a hemodynamically unstable rhythm. In response toconfirming a hemodynamically unstable rhythm, pacemaker 100 withholdsATP therapy delivery.

In some cases, the pacemaker control module 206 may not be limited tousing complete sampling intervals 505 for obtaining activity counts forconfirming a hemodynamically unstable rhythm. For example, an activitycount 512, obtained after the onset 525 of the fast rate and accumulatedover a complete sampling interval 505, may be summed with a partialactivity count 513 that has been reached upon VT detection 524 during anactivity count sampling interval 505. The sum of the complete activitycount 512 and the partial activity count 513 may be compared to a scaledunstable rhythm detection threshold that has been adjusted from theestablished unstable rhythm detection threshold 506 to take into accountthe total time interval over which the summed activity counts 512 and513 were accumulated. To illustrate, if one complete activity count at asampling interval of 2 seconds and one partial activity countaccumulated over 1 second are summed, the total time interval is 1.5activity count sampling intervals. The complete and partial activitycounts may be summed to obtain a 3 second count that is compared to 1.5times the unstable rhythm detection threshold 506.

In another example, an activity count counter may be started upondetecting one or more fast RR intervals such that an activity count isstarted near the onset 525 of the VS events 522 that lead to VTdetection 524. The activity count counter is allowed to run until VTdetection 524 occurs (or until a VT interval counter is reset due to VTdetection criteria not being met at which time it may also be reset).The activity count reached upon VT detection 524 is compared to a scaledthreshold that has been adjusted from the unstable rhythm detectionthreshold 506 based on a ratio of the total time that the activity countwas accumulated during VT detection to the activity count samplinginterval 505 that was used to establish threshold 506. For example, ifan activity count is accumulated over 2.5 seconds during VT detection,the activity count may be compared to 1.25 times the unstable rhythmdetection threshold 506 established based on 2-second activity counts.If the accumulated activity count is less than 1.25 times the unstablerhythm detection threshold 506, a hemodynamically unstable rhythm isconfirmed. In this way, an activity metric may be determined during afast rate for confirming an unstable rhythm that is not limited to anactivity metric sampling interval that is used for determining theactivity metric when a fast rate or VT is not being detected.

ICD 14 senses ventricular events 530 from a received ECG signal anddepending on the particular detection algorithms implemented inpacemaker 100 and ICD 14, ICD 14 is expected to make a VT detection 534at approximately the same time as the VT detection 524 made by pacemaker100. In response to VT detection 534, ICD 14 determines if any ATPpulses are sensed during a delay interval 536. Delay interval 536 may beall or a portion of a capacitor charging time during which high voltagecapacitors are charged in preparing for delivering a shock pulse if oneis needed. Accordingly in some examples, ICD capacitor charging and ICDmonitoring for ATP pulses being delivered by pacemaker 100 may occursimultaneously after VT detection 534. In other examples, capacitorcharging may not begin until after an initial check for ATP pulses ismade. Capacitor charging may begin at the beginning, during orexpiration of a delay interval 536. The delay time 536 may be apredetermined time interval during which ICD 14 monitors for ATP pulsesor may not be predefined and instead be the capacitor charge time. IfATP pulses are detected at any time during the capacitor charge time,the shock pulse 540 is delayed.

In the example shown, capacitor charging is complete upon expiration ofdelay time 536, which may be the required time to charge high voltagecapacitors of therapy delivery module 84 to deliver a programmed shockenergy. Since no ATP pulses are detected during delay time 536 (becausepacemaker 100 has confirmed a hemodynamically unstable rhythm based onthe activity counts 512 and/or 514 and has withheld ATP), ICD 14delivers a shock 540 to terminate the unstable rhythm. In some cases,pacemaker 100 will deliver post-shock pacing pulses 550 if ventricularevents are not sensed due to post-shock asystole or bradycardia. In thisway, shock 540 may be delivered promptly upon completing capacitorcharging after the VT detection 534 and confirmation that pacemaker 100has confirmed a hemodynamically unstable rhythm based on the absence ofATP during the delay interval (or capacitor charging time) 536.

FIG. 8 is a timing diagram 600 illustrating the operations of ICD 14 andpacemaker 100 according to another example. Pacemaker control module 206determines activity counts 602, 610, 612 and 614 (or another activitymetric) at regular activity count sampling intervals 605. The LR setpoint 604 and the unstable rhythm detection threshold 606 are shown bydashed lines.

Pacemaker 100 senses ventricular events 620. VT detection 624 is made inresponse to a series of ventricular sensed events 622 meeting VTdetection criteria, e.g., based on RR intervals. Upon the VT detection624, pacemaker 100 analyzes one or more activity counts, e.g., 610, 612and/or 614, occurring immediately prior to and/or immediately followingVT detection 624. In one example, if at least the most recent activitycount 612 preceding VT detection 624 is greater than the unstable rhythmdetection threshold 606, the VT is detected as a hemodynamically stablerhythm. The pacemaker control module 206 controls pulse generator 202 todeliver ATP pulses 650 to terminate the VT.

In another example, if the most recent two activity counts 610 and 612are both greater than the unstable rhythm detection threshold 606 or atleast one of the most recent two activity counts 610 and 612 is greaterthan the LR set point 604, the VT is determined to be hemodynamicallystable, i.e., potentially treatable with ATP. One or more consecutiveactivity counts 610, 612 immediately preceding the VT detection 624(after onset 625 of the fast rate) and/or one or more activity counts614 immediately after the VT detection 624 may be used in confirming ahemodynamically unstable rhythm. As described above, all of the activitycounts that occur during a series of RR intervals 622 that meet VTdetection criteria, partial activity counts accumulated during VTdetection, and/or one or more activity counts immediately following VTdetection 624 may be used to confirm a hemodynamically unstable rhythm.

ICD 14 senses ventricular events 630 and is expected to make a VTdetection 634 at approximately the same time as the VT detection 624made by pacemaker 100. In response to VT detection 634, ICD 14determines if any ATP pulses are sensed during a delay interval 636.Delay interval 636 may be all or a portion of a capacitor charging timeduring which high voltage capacitors are charged in preparing fordelivering a shock pulse if one is needed. In this example, ATP pulses650 are sensed by ICD 14. ATP is detected by ICD 14 based on the rate ofthe pacing pulse sensed events 640. For example, ICD 14 may detect ATPbased on a rate of pacing pulse sensed events 640 at an expected ATPpulse rate or at a rate faster than the series of VS events 622 that ledto VT detection 634.

In response to detection ATP, ICD processor and control module 80withholds a shock therapy. If capacitor charging has already begun overinterval 636, the ICD processor and control module 80 may wait until ATPpulses 650 are no longer detected and determine if the VT is still beingdetected. The shock therapy may be delivered after ATP if VT is stillbeing detected. If the VT is successfully terminated by ATP pulses 650,the HV capacitors can be discharged through a non-therapeutic load andthe scheduled shock pulse is cancelled.

FIG. 9 is a flow chart 700 of a method for controlling shock delivery byICD 14 in the presence of intracardiac pacemaker 100. At block 702, ICD14 is approaching detection of a shockable, VT rhythm based on VTdetection criteria. For example, the ICD 14 may detect a thresholdnumber of fast RR intervals in a VT interval zone but a required numberof VT intervals for detecting a shockable rhythm may not yet be reached.In another example, an n-second ECG signal may be classified as ashockable segment based on signal morphology and/or RR intervalanalysis, but a required number of shockable segments for detecting ashockable rhythm has not yet been detected.

If shockable rhythm detection criteria have been partially satisfied,the ICD 14 may begin monitoring for ATP pulses. ICD processor andcontrol module 80 determines if ATP is detected at block 406 based onpacing pulse sensed event signals received from sensing module 86meeting ATP detection criteria. If ATP is detected, the RR intervalsprior to ATP detection may be discarded for purposes of detecting ashockable rhythm by ICD 14 at block 706. In some examples, shockablerhythm criteria may include counting a threshold number of VT intervals.In this case, the VT interval counters may be cleared in response todetecting ATP or any detected RR intervals that include ATP pulsedetection are discarded for the purposes of detecting a shockablerhythm.

In another example, if a 3-second segment is detected as a shockablesegment based on morphology analysis, the cardiac signal analyzer 90 isapproaching shockable rhythm detection. At least two out three n-secondsegments may be required to be classified as shockable segments in orderto detect a shockable rhythm. If ATP is detected during any of the threen-second segments, that segment may be discarded for the purposes ofdetecting a shockable rhythm. In this way, shockable rhythm detection,and therefore a subsequent shock, may be delayed by the detection of ATPbeing delivered by the pacemaker 100.

In some cases, ECG-based shockable rhythm detection criteria must befully satisfied after the detected ATP without using any RR intervals orECG signal segments that occurred prior to the detected ATP. In otherexamples, if shockable rhythm detection criteria was partially satisfiedprior to detecting ATP, a reduced number of VT intervals, shockable ECGsegments, or other shockable rhythm detection criteria may be appliedafter the ATP detection to allow more rapid shockable rhythm detectionafter ATP to lead to prompt shock delivery in case the ATP does notterminate the tachyarrhythmia.

At block 708, capacitor charging may be started in response to detectingATP in anticipation that a shock may be needed, even before ICD 14 hasreached a shockable VT rhythm detection. In other examples, capacitorcharging is not started until the ICD detects a shockable rhythm (i.e.,after decision block 710). If shockable rhythm criteria are satisfiedafter detecting ATP, as determined at decision block 710, the ICD 14 mayverify ATP is not being detected at block 714 prior to shock delivery atblock 716.

If ATP is detected by ICD 14 after shockable rhythm detection criteriaare satisfied but before shock delivery, e.g., before capacitor chargingis completed, the ICD 14 may return to block 706 and discard RRintervals and/or ECG signal segments that led to the shockable rhythmdetection. In other examples, redetection criteria are applied afterdetecting the ATP pulses. Shockable rhythm redetection criteria appliedafter an initial shockable rhythm detection and ATP detection mayrequire fewer RR intervals and/or ECG time segments meeting redetectioncriteria than the number of RR intervals and/or ECG time segmentsrequired to make the initial shockable rhythm detection.

If ATP is not detected at block 714, a shock is delivered at block 716by ICD 14 according to programmed shock therapy control parameters. If ashockable rhythm is not detected after ATP detection, as determined atblock 710, a scheduled or expected shock is canceled at block 712.Partially or fully charged high voltage capacitors may be allowed todischarge through a non-therapeutic load.

Thus, various embodiments of a medical device system and method havebeen described for controlling cardiac electrical stimulation therapiesin an IMD system including an intracardiac pacemaker and an ICD.However, one of ordinary skill in the art will appreciate that variousmodifications may be made to the described embodiments without departingfrom the scope of the following claims.

1. An implantable medical device system, comprising: an activity sensorconfigured to produce a signal correlated to patient activity andcomprising heart motion signals; and a cardiac pacemaker comprising: afirst sensing module configured to receive a first cardiac electricalsignal; a pulse generator configured to generate and deliver pacingpulses to a patient's heart via a pair of electrodes; and a controlmodule coupled to the first sensing module, the pulse generator and theactivity sensor and configured to: detect a ventricular tachyarrhythmiafrom the first cardiac electrical signal; determine an activity metricfrom an activity sensor signal; confirm a hemodynamically unstablerhythm based on the activity metric; and control the pulse generator towithhold anti-tachycardia pacing (ATP) in response to confirming thehemodynamically unstable rhythm.
 2. The system of claim 1, wherein thepacemaker control module is further configured to deliver the ATP by thepacemaker in response to not confirming the hemodynamically unstablerhythm based on the activity metric; the system further comprising animplantable cardioverter defibrillator (ICD), the ICD comprising: asensing module configured to receive a second cardiac electrical signal;a therapy delivery module configured to deliver a shock therapy viaelectrodes coupled to the ICD; and a processor and control moduleconfigured to: detect the ventricular tachyarrhythmia from the secondcardiac electrical signal; detect from the second cardiac electricalsignal at least one ATP pulse delivered by the pacemaker; and withhold ashock therapy response to detecting the at least one ATP pulse.
 3. Thesystem of claim 1, wherein the pacemaker control module is furtherconfigured to confirm the hemodynamically unstable rhythm by: comparingthe activity metric to a normal resting range of the activity metriccorresponding to a resting state of the patient; and confirming thehemodynamically unstable rhythm in response to the activity metric beingless than the normal resting range.
 4. The system of claim 1, whereinthe pacemaker control module is further configured to: establish a lowerrate set point as a maximum value of the activity metric during apatient resting state; establish an unstable rhythm threshold based onthe lower rate set point; wherein confirming the hemodynamicallyunstable rhythm comprises comparing the activity metric to the unstablerhythm threshold.
 5. The system of claim 2, wherein: the pacemakercontrol module is further configured to: compare the activity metric toan unstable rhythm threshold that is less than an expected resting rangeof the activity metric; confirm the detected ventricular tachyarrhythmiaas a hemodynamically unstable rhythm in response to the activity metricbeing less than the unstable rhythm threshold; withhold the ATP inresponse to confirming the hemodynamically unstable rhythm; the ICDprocessor and control module is further configured to: responsive todetecting the ventricular tachyarrhythmia, determine from the secondcardiac electrical signal an absence of ATP pulses delivered by thepacemaker; and control the therapy delivery module to deliver a shocktherapy in response to determining the absence of ATP pulses.
 6. Thesystem of claim 5, wherein the ICD processor and control module isfurther configured to: determine the absence of the ATP pulses bymonitoring for at least one ATP pulse during a shock delay interval; andcontrol the therapy delivery module to deliver the shock therapy afterexpiration of the shock delay interval in response to determining theabsence of the ATP pulses during the shock delay interval.
 7. The systemof claim 6, wherein the ICD processor and control module is configuredto control the therapy delivery module to charge a high voltagecapacitor during at least a portion of the shock delay interval.
 8. Thesystem of claim 2, wherein the ICD processor and control module isconfigured to control the therapy delivery module to charge a highvoltage capacitor in response to detecting the at least one ATP pulse.9. The system of claim 2, wherein the ICD processor and control moduleis further configured to: delay the withheld shock therapy in responseto sensing the at least one ATP pulse; and control the therapy deliverymodule to deliver the delayed shock therapy in response to redetectingthe ventricular tachyarrhythmia after detecting the at least one ATPpulse and no longer detecting ATP pulses.
 10. The system of claim 1,wherein the pacemaker control module is configured to determine theactivity metric by determining an activity count from an accelerometersignal.
 11. The system of claim 1, wherein the pacemaker control moduleis configured to determine the activity metric by: determining aplurality of activity metrics at an activity metric sampling interval;identifying from the plurality of activity metrics at least one of amost recent activity metric prior to detecting the ventriculartachyarrhythmia and an earliest occurring activity metric afterdetecting the ventricular tachyarrhythmia.
 12. The system of claim 1,wherein the pacemaker control module is further configured to: determinea signal reliability metric from the activity sensor signal; compare thesignal reliability metric to signal reliability criteria; and withholdthe ATP if the signal reliability metric does not meet the signalreliability criteria.
 13. The system of claim 12, wherein the pacemakercontrol module is further configured to: determine the signalreliability metric by determining a plurality of activity metrics fromthe activity sensor signal during a time interval preceding theventricular tachyarrhythmia detection; and determine whether the signalreliability metric meets the signal reliability criteria by comparing adistribution of the plurality of activity metrics to an expectedactivity metric distribution.
 14. The system of claim 12, wherein thepacemaker control module is further configured to: determine the signalreliability metric by determining a rate of heart motion signals fromthe activity sensor signal during a time interval preceding theventricular tachyarrhythmia detection; determine a heart rate from thefirst cardiac electrical signal; and determine that the signalreliability metric meets the signal reliability criteria by comparingthe determined rate of heart motion signals to the heart rate.
 15. Thesystem of claim 1, wherein the activity sensor comprises an intracardiacaccelerometer.
 16. The system of claim 2, further comprising: anextravascular electrical lead coupled to the ICD and carrying aplurality of electrodes; and wherein the pacemaker is configured to bewholly implanted within a chamber of a patient's heart and comprises ahousing enclosing the activity sensor, the pacemaker sensing module, thepulse generator and the pacemaker control module.